Mortise and Tenon Calculator — Joint Dimensions

The traditional rule is one-third of the stock thickness. For a 45mm thick rail, the tenon should be about 15mm thick. This leaves equal-sized shoulders on each face, providing good glue surface and resistance to racking.

For a through tenon, the length equals the full width of the mortise piece. For a stub (blind) tenon, the length should be about two-thirds of the mortise piece width. Never make a stub tenon so long that it weakens the mortise piece.

A haunched tenon has a small step cut into one edge. The haunch fills a groove (typically for a panel) and prevents the joint from twisting. It is commonly used in frame-and-panel door construction.

The mortise should match the tenon thickness exactly for a snug fit. It should be 1–2mm deeper than the tenon length to allow space for excess glue. Some woodworkers make the tenon 0.1–0.2mm thicker and pare to fit.